1. Field of the Invention
The present invention relates to, for example, an antenna coil device, provided on a busbar of defogging heating elements of a rear window glass of a vehicle. More particularly, the present invention relates to, for example, an antenna coil device for preventing noise from being superimposed on radio broadcast signals received by an automobile glass antenna, or for improving receiving sensitivity.
2. Description of the Prior Art
An automobile glass antenna has been known that uses heating elements of a defogger provided in a rear window or antenna element provided around the heating elements as an antenna for radio broadcast or TV broadcast in order to receive radio broadcast or TV broadcast in a vehicle.
Radio wave signals of radio broadcast or TV broadcast received by such a glass antenna for a vehicle is affected by noise included in an output of a battery for supplying power to the heating elements of the defogger, or the received radio wave signals which leak through a feeder to a body or a DC power supply.
For this reason, a coil device is connected between defogging heating elements of a rear window glass of a vehicle and the body or the DC power supply to increase an impedance between a busbar and the body or the busbar and the DC power supply, thereby preventing current leaks. Further, in order to prevent the received radio wave signals from leaking from the feeder for the defogger to the body, a coil held by a resin member is connected and secured onto the busbar by soldering or the like.
Known conventional antenna coil devices for removing noise in radio wave signals of radio broadcast or TV broadcast received by an automobile glass antenna are described in, for example, Japanese Unexamined Patent Publication Nos. HEI 8-335820 and HEI 9-213528.
In the above described automobile glass antenna coil device, both ends of a lead wire of a wound coil body (hereinafter simply referred to as a coil) are each connected to corresponding terminal boards, and a DC current for a defogger input through the terminal boards passes through the coil. Generally, input/output positions of external signals to the two terminal boards are on the same side (referred to as a front side of the coil), and thus one terminal board, to which a back end of the coil is connected, axially extends from the front side to a back side of the coil, and is placed close to and opposite the other terminal board in a midway region.
The coil and the terminal boards are made of high conductive metal, and in order to prevent heating in the coil device, the cross-sectional area of the coil is designed to be large and the terminal boards are designed to be thick.
Such a coil device is mounted to an automobile, and strength at the time of mounting needs to be ensured to provide a vibration-resistant structure, and thus each terminal board is as wide as the coil and surrounded by a resin member to increase strength.
In the above described automobile glass antenna coil device, the two terminal boards are placed close to and opposite each other, thereby causing stray capacitance between the terminal boards.
However, as described above, each terminal board is as wide as the coil and has a large area in order to increase the strength, thus causing large stray capacitance proportional to an area of an opposite region between the terminal boards.
On the other hand, a self resonance frequency f is expressed by 1/(2π(LC)1/2) where C is a stray capacitance component and L is an inductance component of the coil (inductor), and the self resonance frequency f decreases as the stray capacitance component C increases.
Therefore, in the coil device, the terminal board is as wide as the coil and has the large area to reduce the self resonance frequency f, and reduce an impedance in, for example, a desired FM frequency band, especially in a high frequency band thereof, thereby causing difficulty in removing noise, and reducing sensitivity of an antenna.
For the device described in Japanese Unexamined Patent Publication No. HEI 8-335820, a stray capacitance component C increases as described above to require a reduction in a inductance component L, and to thus reduce an impedance, thereby causing difficulty in removing noise and improving sensitivity of an antenna.
For the device described in Japanese Unexamined Patent Publication No. HEI 9-213528, a dielectric is provided, and a resonance frequency is adjusted by changing the dielectric. However, a portion on which the dielectric is provided is extremely weakened, and providing a reinforcing member for reinforcing the portion increases costs resulting from an increase in the number of parts, and also causes a wide range of performance variations.
The invention is achieved in view of such circumstances, and has an object to provide an antenna coil device that has a simple structure, prevents an increase in stray capacitance caused between opposite terminal boards to prevent a reduction in a self resonance frequency, and improves sensitivity of an antenna while ensuring a noise removing function in a desired frequency band of a received signal, even if each terminal board of the coil device is wide and has a large area.
The above described problems are caused by an increase in a surface area of a conductor of opposite terminal boards of a coil device.